The low viscosity of dense CO.sub.2 relative to liquid hydrocarbons at similar conditions prevents it from being effectively used as a displacing fluid in porous media. The CO.sub.2 channels through the porous media since it has a high mobility, bypassing much of the hydrocarbon phase rather than displacing it. If the CO.sub.2 mobility could be decreased to the same value as the hydrocarbons, the channeling would be inhibited and the recovery of hydrocarbons increased.
The low CO.sub.2 viscosity also inhibits its ability to transport small solid particles into formation fractures. These small particles, designed to prop up the fracture caused by the high pressure injection of CO.sub.2, must travel as far as possible into the fracture before it collapses in order to increase the permeability of the reservoir. A significant increase in CO.sub.2 viscosity would decrease the settling velocity of the particles, allowing the CO.sub.2 to transport the particles further into the fracture.
Three methods of reducing the mobility of liquid or supercritical carbon dioxide in porous media include the alternate injection of water, formation of CO.sub.2 emulsions or foams and the direct thickening of carbon dioxide. The alternate injection of an aqueous phase reduces the CO.sub.2 saturation and therefore the CO.sub.2 relative permeability. Emulsions in which the liquid or supercritical CO.sub.2, where the interior CO.sub.2 phase is separated by aqueous lamallae which contain a small concentration of surfactant, can greatly diminish the CO.sub.2 mobility since the size of the pockets of CO.sub.2 are of the order of magnitude of the pores. The direct viscosity enhancement of CO.sub.2 is a proposed method of CO.sub.2 mobility reduction in which the CO.sub.2 viscosity is greatly enhanced by the dissolution of small concentrations of a thickening agent directly into the carbon dioxide.
Alternate injection of water and gas, WAG, has been used successfully not only in the CO.sub.2 process, but also in other gas displacement processes. Klins, M., Carbon Dioxide Flooding--Basic Mechanisms and Project Design, IHRDC, Boston (1984). Stalkup, F., Miscible Displacement, SPE Monograph No. 9, SPE, New York (1983). Although the mobility ratio of CO.sub.2 /water to the fluid being displaced is reduced, it usually remains unfavorable. Furthermore, the injection of water introduces several operational difficulties and increases the time required to inject the entire CO.sub.2 slug and, therefore, the duration of the oil recovery project. Slugs of carbon dioxide emulsions, sometimes referred to as foams, have displayed extremely low mobilities in lab tests, but difficulties are encountered in retaining their integrity when they contact crude oil. Stalkup, F., Miscible Displacement, SPE Monograph No. 9, SPE, New York (1983). Heller, J. and Taber, J., "Mobility Control for Co.sub.2 Floods--A Literature Survey, Topical Report," DOE/MC/10689-3 (Oct. 1980). Heller, J. and Taber, J., "Development of Mobility Control Methods to Improve Oil Recovery by Co.sub.2, Final Report," DOE/MC/10689-17 (Nov. 1983). Heller, J., Cheng, L. and Kuntamukkula, M., "Foam-Like Dispersions for Mobility Control in CO.sub.2 Floods," -SPE 11233, presented at the 57th Annual Fall Technical Conference and Exhibition of the SPE of AIME, New Orleans, LA (Sept. 26029, 1982). Bernard, G., Holm, L. and Harvey, C., "Use of Surfactant to Reduce Co.sub.2 Mobility in Oil Displacement," SPEJ, Aug. 1980, pp. 282-292. Wang, G., "A Laboratory Study of CO.sub.2 Foam Properties and Displacement Mechanism," SPE/DOE 12645, presented at the SPE/DOE Fourth Symposium on Enhanced Oil Recovery, Tulsa, OK (Apr. 15-18, 1984). These slugs have been proposed as a means of not only displacing oil, but also plugging highly permeable zones. The direct thickening of CO.sub.2 could provide a means of lowering thickening of CO.sub.2 mobility and achieving a favorable mobility ratio (less than one) without introducing large amounts of water or encountering the problems associated with the generation or propagation of a foam. Heller, J. and Taber, J., "Development of Mobility Control Methods to Improve Oil Recovery by Co.sub.2, Final Report," DOE/MC/10689-17 (Nov. 1983).
Viscosity measurements of carbon dioxide-direct thickener mixtures were reported by Orr, F. M., Jr., J. P. Heller and J. J. Taber, "Carbon Dioxide Flooding for Enhanced Oil Recovery: Promise and Problems,: J. A. O. C. S., Vol. 59, No. 10, (Oct., 1982), p. 810A. Therein it is stated that a polymer which could dissolve at low concentrations and increase the viscosity by a factor of 20 would be needed to make the process economically feasible. However, viscosity enhancements of up to only twenty percent were found in their preliminary experiments. The atactic, straight chain polymers of relatively low molecular weight were soluble in CO.sub.2, while the higher molecular weight and isotactic ones were insoluble. The small changes in carbon dioxide's viscosity were due to several factors, the foremost being its inability to dissolve high molecular weight polymers. Plans were also mentioned of continuing the search for more effective compounds among the product lists of manufacturers, and to initiate the synthesis of new polymers may be necessary.
Heller, J. J. and J. J. Taber, "Development of Mobility Control Methods to Improve Oil Recovery by CO.sub.2 --Final Report," U.S. Dept. of Energy Report DOE/MC/10689-17, Nov. 1983 later reported that no CO.sub.2 -thickening polymer was found among the current products of any manufacturer. However, they believed that significant progress had been made in the characteristics of polymers which enhance their solubility in CO.sub.2, perhaps enabling the synthesis of new polymers for this purpose.
In 1983, Heller, J. P., Dandge, D. K., Card, R. J. and Donarume, L. G., "Direct Thickeners for Mobility of CO.sub.2 Floods," SPE 11789, S.0.E. of A.I.M.E., June 1983 further discussed the effect of polymer structure and properties on solubility in CO.sub.2. They found that halogens, aldehydes, ring systems with unsaturation in the chain backbone and aromaticity in general were not desirable for a polymer to be soluble in CO.sub.2. Similarly, insolubility was found when there was the presence of amide, ester, carbonate and hydroxyl groups in the polymer backbone. Soluble polymers generally had solubility parameters less than eight, but the authors concluded that the compounds' solubilities could not be described with this parameter alone. It was also found that higher molecular weight polymers were much more effective, on a weight concentration basis in increasing the viscosity of carbon dioxide. From this, they concluded that if higher molecular weight polymers could be synthesized which are soluble in carbon dioxide, larger viscosity increases could be achieved with smaller concentrations. The majority of soluble polymers had molecular weights under 6000.
Heller, J. P., Orr, F. M., Jr., and Watts, R. J., "improvement of CO.sub.2 Flood Performance," U.S. Dept. of Energy Report DOE/BCO-85/1, Dec. 1984, p. 50 have also investigated the feasibility of using tri-alkyltin fluorides to increase CO.sub.2 viscosity. This compound can form associating polymers in propane, butane and hexane which are capable of significantly increasing the fluid viscosity. These non-polar fluids do not interfere with the association between the tin and fluorine of adjacent tri-alkyltin fluoride molecules. CO.sub.2, a fluid with no dipole moment, was not able to dissolve tri-butyltin fluoride, the only commercially available tri-alkyltin fluoride, to a great enough extent to induce any notable increase in the viscosity. Current research by the same investigator is directed at the effects of other types of hydrocarbon groups, rather than butyl, on the solubility of this organometallic compound in carbon dioxide.
Recent work by Terry, R. E., Zaip, A., Angelos, C. and Whitman, D. L., "Polymerization in Supercritical CO.sub.2 to Improve CO.sub.2 /Oil Mobility Ratios," SPE 16270, SPE of AIME, June 1983 has concentrated on synthesizing carbon dioxide soluble polymers in-situ. Using an apparatus that simulates reservoir conditions, the authors found that light olefins can be readily polymerized in such an environment using commonly available initiators. However, no apparent viscosity increases have been measured, since the solubility of the resultant polymer is low.
Dandge, D. K. and Heller, J. P. "Polymers for Mobility Control in CO.sub.2 Flods," paper SPE 16271 have also reported success in synthesizing new carbon dioxide soluble polymers from high alpha-olefins, but none have yet been found which satisfactorily enhance the CO.sub.2 viscosity.
The present invention involves the use of semi-fluorinated alkanes with CO.sub.2 to increase the viscosity of CO.sub.2. Semi-fluorinated alkanes have been shown to form gels when dissolved in alkanes such as decane and octane, probably due to resultant formation of a microfibrillar morphology but hereto for have not been known to form gels in CO.sub.2. This gel phase in decane and octane results when the mixture is heated above the melting point of the F(CF.sub.2).sub.N (CH.sub.2).sub.M H compound, and then cooled. Tweig, R. J., Russell, T. P., Siemens, R. and Rabolt, J. F., Observations of a Gel Phase in Binary Mixtures of Semifluorinated n-Alkanes with Hydrocarbon Liquids," Macromolecules, Vol. 18, No. 6 (1985) p. 1361.